"Tango down" is an expression used by the US Special Forces when they have killed an enemy.

The CIA website at cia.gov was offline at the time of writing, and a spokesperson said the intelligence agency was looking into the reports.

"We are aware of the problems accessing our website, and are working to resolve them," she said.

The website was restored shortly thereafter.

Frequent attacks

Anonymous last month briefly disabled the websites of the US justice department and the Federal Bureau of Investigation offline using distributed denial of service attacks (DDoS), a common technique used by the collective where multiple sources bombard the target website.

There was no immediate explanation from Anonymous for the targeting of the CIA site.

It also released a recording of a telephone call between policing agencies of numerous countries discussing their hacking activities.

Those attacks were in retaliation for the US shutdown of file-sharing site Megaupload.

Most Anonymous cyber attacks are distributed denial-of-service attacks in which a large number of computers are commanded to simultaneously visit a website, overwhelming its servers.